1) Field of the Invention
The present invention relates to an active-matrix-type image display apparatus controlling brightness of a current-controlled light emitting element, and more particularly, to an image display apparatus that suppresses a decrease in refresh rates to perform high-quality image display.
2) Description of the Related Art
An organic electro-luminescence (EL) display apparatus using an organic light-emitting-diode (LED) that emits light autonomously is getting an attention as a next generation image display apparatus, because it does not require a back light that is necessary in a liquid crystal display (LCD) apparatus, which makes it most suitable for reducing thickness of the apparatus, and does not have any limitation in the angle of visibility. Unlike the liquid crystal display apparatus in which a liquid crystal cell is controlled by a voltage, the organic LED used for the organic EL display apparatus has a mechanism that the brightness of each light emitting element is controlled by a current.
In the organic EL display apparatus, a simple (passive) matrix type and an active matrix type can be employed as a drive system. The former has a simple configuration, but has a problem of realization of a large and high definition display. Therefore, recent research and development on the organic EL display apparatus is focused on the active matrix type image display apparatus that controls electric current flowing in a light emitting element in a pixel by a driver element having a device such as a thin film transistor (TFT) provided in the pixel.
The driver element is directly connected to the organic LED, and becomes ON at the time of displaying an image, to supply the current to the organic LED, so that the organic LED emits light. Therefore, when the image display apparatus is used for long time, and threshold voltage of the TFT included in the driver element fluctuates, even when the voltage supplied into the pixel is constant, the current flowing through the driver element fluctuates, and hence the current flowing through the organic LED also fluctuates. Therefore, the emission brightness of the organic LED becomes nonuniform, thereby deteriorating the image quality of the displayed image.
To cope with the problem, an image display apparatus having a compensation circuit that makes up for the fluctuations in the threshold voltage of the driver element is necessary. FIG. 16 a circuit diagram of a pixel circuit having such compensation circuit according to a conventional technology. The conventional image display apparatus includes a data line 310 for supplying data voltage corresponding to the emission brightness and zero voltage, a select line 320, a reset line 330, a merge line 340, and a power line VDD. Further, the image display apparatus includes a TFT 360, a TFT 365, a TFT 370, a TFT 375, a capacitor 350, a capacitor 355, and an organic LED 380. The TFT 365 serves as a driver element, and the capacitor 350 and the capacitor 355 are connected to a gate electrode of the TFT 365. A predetermined voltage from among the data voltages charged in the capacitor 350 and the capacitor 355 becomes the gate-source voltage of the TFT 365, and the current corresponding to the gate-source voltage flows through the TFT 365.
FIGS. 17A to 17D are circuit diagrams for illustrating operating processes of the pixel circuit shown in FIG. 16. In the pixel circuit in the conventional technology, the organic LED 380 emits light at a light emitting step after the data voltage is written through a zero voltage applying step and a threshold voltage detecting step. Solid line in FIGS. 17A to 17D indicates a current flowing region, and broken line indicates a non-current flowing region.
FIG. 17A depicts the zero voltage applying step. The voltage applied to the data line 310 is changed from the data voltage to the zero voltage. Since, when a data driver controlling the applied voltage to the data line 310 changes the applied voltage to the data line 310, a certain period of time is required in the pixel circuit away from the data driver until the applied voltage becomes stable, the zero voltage applying step is necessary. After the applied voltage to the data line 310 is stabilized at the zero voltage, the zero voltage is supplied to the capacitor 350 by setting the select line 320 to a low level and the TFT 360 to the ON state.
FIG. 17B depicts the threshold voltage detecting step. By setting the reset line 330 to a low level and the TFT 370 to the ON state, the gate and the drain of the TFT 365 become conductive to each other. The TFT 360 becomes the ON state, and the zero voltage is supplied from the data line 310, to the capacitor 350. By setting the merge line 340 to a low level, the transistor 375 becomes the ON state, so that the current flows to the TFT 365. When the gate-drain voltage of the TFT 365 becomes the threshold voltage, the TFT 365 becomes the OFF state, thereby finishing detection of the threshold voltage. During the threshold voltage detecting step, the zero voltage is applied to the data line 310.
Then, control proceeds to a data writing step shown in FIG. 17C. In this case, the voltage applied to the data line 310 is changed to the data voltage. After the applied voltage to the data line 310 is stabilized at the data voltage, the select line 320 becomes a low level, and the TFT 360 becomes the ON state, and hence the data voltage is supplied from the data line 310 to the capacitor 350. Thereafter, the TFT 360 becomes the OFF state, to finish the data writing step, and control proceeds to the light emitting step shown in FIG. 17D.
As shown in FIG. 17D, by setting the merge line 340 to the low level and the TFT 375 to the ON state, the current corresponding to the gate-source voltage flows to the TFT 365, so that the organic LED 380 emits light. Since the gate-source voltage of the TFT 365 includes the threshold voltage detected at the threshold voltage detecting step, even when fluctuations occur in the threshold voltage of the TFT 365, desired current can be allowed to flow to the organic LED 380, regardless of the deterioration of the TFT 365 (see, for example, U.S. Pat. No. 6,229,506 (FIG. 3)).
However, in the pixel circuit shown in FIG. 16, the time required for displaying one screen increases, thereby causing a problem of decrease in refresh rate, the number of times for displaying the screen in one second. The decrease in the refresh rate is caused by the fact that the data line 310 supplies the data voltage and the zero voltage.
In order to detect the threshold voltage stably, the state in which the zero voltage is supplied to the capacitor 350 is required. As described above, after the applied voltage to the data line 310 is changed from the data voltage to the zero voltage by the data driver, the zero voltage is supplied from the data line 310 to the capacitor 350. However, certain time is required for the applied voltage to the data line 310 to be changed from the data voltage to the zero voltage and stabilized at the zero voltage. Therefore, the zero voltage applying step is conventionally necessary. Further, certain time is also required until the applied voltage to the data line 310 is changed from the zero voltage to the data voltage and stabilized at the data voltage. Therefore, starting of the data writing step takes time, too.
In the pixel circuit away from the data driver, when the voltage applied to the data line 310 is changed, more time is required until such a voltage becomes stable, as compared with a pixel circuit closer to the data driver. Further, when a signal delay occurs in the data line 310, more time is required for supplying the voltage from the data line 310.
In the image display apparatus according to the conventional technology, it is necessary to take the period until the applied voltage to the data line 310 becomes stable into consideration, in order to start the threshold voltage detecting step and the data writing step. Therefore, long time is necessary until the data writing step finishes, and hence the light emitting time cannot be ensured, and the refresh rate drops inevitably. Particularly, in the high definition image display apparatus, since it is necessary to reduce the time until the data writing step finishes, high-definition image quality cannot be achieved with the image display apparatus according to the conventional technology. Furthermore, since the threshold voltage detecting step has to be shortened to keep the optimum value of the refresh rate, the fluctuations in the threshold voltage of the driver element cannot be compensated sufficiently, thereby making it difficult to keep the uniformity in the image quality.